1. Field of the Invention
Embodiments of the invention relate to semiconductor fabrication, and in particular, to methods of eliminating pattern deformation in semiconductor devices.
2. Related Art
The use of amorphous carbon film as part of a hardmask stack for patterning MOSFET features has been found to be beneficial due to the ease with which amorphous carbon may be patterned and the high selectivity of amorphous carbon relative to typically used capping or protective materials such as silicon oxide, silicon nitride and silicon oxynitride. FIG. 1 shows a structure including amorphous carbon that may be used in the formation of a MOSFET. The structure includes a semiconductor substrate 2 having field oxides 4 that bound source/drain regions of a MOSFET. A layer of a gate insulating material 6 such as SiO2 is formed over the substrate. A layer of a gate conductive material 8 such as doped polysilicon is formed over the substrate and will be patterned to form a gate line of the MOSFET. Formed over the gate conductive layer 8 is a hardmask stack including an amorphous carbon layer 10 and a capping material layer 12 such as SiON. A photoresist mask 14 for defining the pattern of the gate is formed on the SiON capping material layer 12. During processing, a first etch is used to transfer the photoresist mask pattern to the SiON layer, a second etch is used to transfer the SiON mask pattern to the amorphous carbon layer, a third etch is used to remove oxide from the surface of the gate conductive layer, and a further etches are performed to etch the underlying gate conductive layer using the SiON and amorphous carbon patterns as a hardmask.
One problem with the structure of FIG. 1 is that the amorphous carbon material has relatively poor selectivity with respect to the polysilicon gate conductive material during the polysilicon etch, and as a result the amorphous carbon is also etched during etching of the polysilicon, resulting in degradation of the transferred pattern. A proposed solution to this problem is to dope the amorphous carbon with nitrogen, which enhances its selectivity with respect to polysilicon.
However, the nitrogen doping technique creates other problems that become more significant as device dimensions are reduced. One problem involves poisoning of the photoresist with nitrogen from the amorphous carbon layer. Poisoning is enabled by pinholes in the SiON cap layer that randomly occur during SiON deposition. The pinholes extend partly or entirely through the SiON layer, enabling nitrogen dopant from the amorphous carbon to diffuse into the photoresist. Poisoned photoresist is difficult to remove by conventional developing techniques and therefore the poisoned photoresist degrades the quality of the photoresist mask. As SiON cap layers become thinner, the poisoning problem becomes more pronounced.
A second problem of amorphous carbon is delamination of etched amorphous carbon from the underlying polysilicon. FIGS. 2a and 2b illustrate this problem. FIG. 2a shows a top view of a patterned amorphous carbon line. The line is subject to compressive forces 16 resulting from differences in the thermal expansion coefficients of amorphous carbon, polysilicon and SiON. As the width of the line decreases relative to its length, the compressive forces along the length of the line become significantly greater than those across the width of the line. So long as a SiON top layer is present on the amorphous carbon line, the compressive forces do not deform the line. However, during typical processing, an etch for removing oxide from the polysilicon layer is performed after patterning the amorphous carbon, and this etch typically removes most or all of the SiON overlying the amorphous carbon line. At that point the internal compressive forces of the amorphous carbon are no longer restrained, and the amorphous carbon delaminates from the underlying polysilicon and may assume a xe2x80x9csquigglexe2x80x9d pattern as shown in FIG. 2b that effectively lengthens the line to relieve compressive stress. This pattern will be reproduced in the polysilicon upon further etching, resulting in a deformed gate line. The severity of this problem is enhanced by nitrogen doping.
Consequently, there is a need for methods that reduce pattern deformation and photoresist poisoning while maintaining the desirable etch selectivity properties of nitrogen doped amorphous carbon.
It is an object of the present invention to reduce pattern deformation in semiconductor device fabrication.
In accordance with one preferred embodiment of the invention, a hardmask stack is comprised of alternating layers of doped amorphous carbon and undoped amorphous carbon. The undoped amorphous carbon layers serve as buffer layers that constrain the effects of compressive stress within the doped amorphous carbon layers to prevent delamination. The stack is provided with a top capping material layer. The layer beneath the capping material layer is preferably undoped amorphous carbon to reduce photoresist poisoning.
In accordance with a second preferred embodiment, a hardmask stack is comprised of alternating layers of a capping material and amorphous carbon. The amorphous carbon layers may be doped or undoped. The capping material layers serve as buffer layers that constrain the effects of compressive stress within the amorphous carbon layers to prevent delamination. The top layer of the stack is formed of the capping material. The layer beneath the capping layer is preferably undoped amorphous carbon to reduce photoresist poisoning. The lowest layer of the hardmask stack is preferably amorphous carbon to facilitate easy removal of the hardmask stack from the underlying materials by an ashing process.